This invention relates to an intermediate seal comprising a beryllium body hermetically sealed to another metallic body formed of a metal the coefficient expansion of which is approximately that of beryllium. The invention also relates to a method for producing such seals.
Intermetallic seals comprising beryllium bodies hermetically joined to other beryllium or other metallic bodies are well-known in the art.
Hanks et al, U.S. Pat. No. 3,090,117, discloses a method for joining together beryllium metal parts by brazing in which a silver aluminum alloy brazing metal is employed and a lithium halide flux is used during the brazing operation. A major difficulty with this method is that due to the reactivity between the lithium and the beryllium the seals have rather a short life span particularly at high temperatures.
Atkinson, U.S. Pat. No. 3,083,451 shows a method of producing a seal between a beryllium and a base metal such as a Monel metal by brazing. Preferably employing a silver copper palladium alloy as a brazing alloy. The brazing procedure is carried out in several steps. The assembly of beryllium metal, brazing solder and base metal is heated in a vacuum first to a temperature between 830.degree. C. and 840.degree. C. and then to a temperature of 910.degree. C. and then rapidly cooled to a temperature of 700.degree. C.
The problem with this method is that it is a complex method requiring several stages of heating and cooling. In addition, as shown in column 2, lines 49-51 the resultant joints are not usable at temperatures above 700.degree. C.
Atkinson, U.S. Pat. No. 3,105,294 shows a method of brazing beryllium in which the beryllium is first etched in an atmosphere of cracked ammonia and then treated in a hot cyanide bath. After which the layer of copper is electroplated onto the etched surface and the copper-plated region is then joined to a base metal frame by brazing in a vacuum or hydrogen atmosphere at a temperature above that of the liquidus temperature of the solder.
The defects in this method is that it requires use of the highly toxic and environmentally dangerous cyanide bath. In addition it is necessary that the brazing be carried out at high temperatures in order to complete the bond between the beryllium surface and the electro deposited copper layer.
Adams et al, U.S. Pat. No. 3,420,978 shows a method of brazing beryllium to another beryllium body or another material in which a thin layer of zirconium or titanium is vacuum deposited on the surface of beryllium and a high purity aluminum is employed as a brazing material, the brazing being carried out at temperatures between 1550.degree. F.-1650.degree. F. and preferably at 1730.degree. C.
Here too the problem exists that due to the fact that since a good tight bond is not developed between the titanium and zirconium layer and the beryllium surface during the vacuum deposition process it is necessary to bring the temperature of the surface to a very high temperature during the brazing operation in order to form a tight bond between the beryllium surface and the titanium or zirconium layer and in order that the brazed bond be stable at extremes of temperature.
Herman, U.S. Pat. No. 3,779,721, shows a method of joining two beryllium bodies by brazing in which successive layers of silver and nickel are deposited on the beryllium surfaces. Here too brazing is carried out at a high temperature for example 1490.degree. C.